The invention relates generally to crystals that allow selected wavelengths of light to pass therethrough, and more particularly, to optical band-pass or rejection filters that operate in the ultraviolet (UV) spectrum.
A variety of optical devices utilize optical band-pass filters or rejection filters, which serve to reject unwanted wavelengths and permit only wavelengths belonging to a selected spectral band to pass therethrough. Crystals that serve as rejection or band-pass filters in the UV portion of the spectrum can be used for various applications, including solar-blind optical systems and missile warning systems. The following references discuss crystals and/or missile detection systems and are incorporated herein by reference: U.S. Pat. Nos. 5,742,428, 5,788,765 and 5,837,054; and MHC Pryce et al, xe2x80x9cLow-temperature Absorption of Nickel Fluorosilicate Crystalsxe2x80x9d, Philos. Mag., 10, 447 (1964).
The light emitted from the exhaust of a jet engine of a missile comprises a broad spectrum, ranging from the infrared to the UV. Thus, a UV filter is needed to enable the system to distinguish this source of UV light from other natural sources of UV energy, such as the sun, moon and stars.
Crystals of nickel sulfate hexahydrate (chemical formula NiSO4.6H2O) have been used as components for such UV filters. In the UV portion of the spectrum, this crystal is transparent at wavelengths less than 300 nm and absorbs light having wavelengths greater than 350 nm. However, such crystals have drawbacks. For example, it is advantageous for an optical system used to detect the UV signal characterizing the jet engine of a missile to operate at various climate conditions, including those associated with elevated temperatures, which might be experienced in the desert. Also, temperatures of 85xc2x0 C. or higher and low relative humidity conditions can be reached when aboard an aircraft, armored vehicle or a ship, as a combined result of both solar heating and engine-generated heat. Thus, the crystal should have not only proper spectral transmission but thermal stability.
The low thermal stability of NiSO4.6H2O crystals can represent a disadvantage. When heated above 60-65xc2x0 C., the material begins to lose its water of crystallization. This limits its use in UV filters. Accordingly, it is desirable to provide a crystal having optical properties similar to that of NiSO4.6H2O, but having greater thermal stability.
Generally speaking, in accordance with the invention, crystals are provided having good optical transmission at wavelengths below 300 nm and strong optical absorption at wavelengths greater than 350 nm as well as high thermal stability. The specific thermal stability is dependent on the relative humidity. At a relative humidity of 40%, the crystals are stable at temperatures up to 90xc2x0 C. At a relative humidity of greater than 60%, the crystals will have a thermal stability greater than 110xc2x0 C. In an enclosed environment, the thermal stability of the crystal is higher than 125xc2x0 C. An example of such a crystal is ammonium nickel sulfate, having the formula (NH4)2Ni(SO4)2.6H2O. Optical devices including missile detection systems are also provided.
Accordingly, it is an object of convention to provide an improved UV transmission crystal.
Another object of the invention is to provide a UV transmission crystal having high thermal stability.
Another object of the invention is to provide an improved missile detection system.
As to other objects and advantages of the invention, will impart the obvious and will impart the apparent from the specification and drawings.
The invention accordingly comprises a product possessing the features, properties and relation of components which will be exemplified in the products hereinafter described, and the scope of the invention will be indicated in the claims.